Tyrosine sulfate (Tyr-S) represents another convalent modification of protein tyrosine residues which has been little studied due to methodological difficulties in detection and quantification. The chemical stability of the sulfate ester bond to tyrosine was studied in chemically synthesized and purified Tyr-S. The stability of Tyr-S to base was found to enable the recovery of 90% of the Tyr-S under conditions which yield complete protein hydrolysis. Secondly, it was found that Tyr-S could be identified and quantified on the amino acid analyzer. Base hydrolysis of a protein containing Tyr-S yielded measurable quantities of Tyr-S using a sensitive amino acid analyzer. The identification of Tyr-S in proteins is now a straightforward methodology. Spectral studies of Tyr-S after HPLC purification show a unique UV spectra different from phosphotyrosine. Spectral characterization of Tyr-S in proteins by a nondestructive method may be possible through sophisticated spectral analyses.